Why doesn't Mars have a magnetic field?

A planet's global magnetic field doesn't come from a buried magnet, but from an engine: molten metal moving within the core, which as it circulates generates electric currents and, with them, a field. Mars has a dense core of iron, nickel and sulphur, with an estimated radius between 1,500 and 2,100 kilometres, but that engine stopped working. A small planet loses its internal heat before a large one does, and without that heat the convection that kept the dynamo running ceases.

That the field once existed is not a guess. NASA notes that, although Mars has no global magnetic field today, there are strongly magnetized regions of the crust in the southern hemisphere, carrying traces of a field from 4 billion years ago. These are rocks that cooled while the dynamo was still active and were fixed in place as witnesses: a magnetic memory scattered in patches, rather than a field wrapping the whole planet.

The consequence can be measured. Observations from the MAVEN mission indicate that the solar wind strips gas from the Martian atmosphere at a rate of about 100 grams per second, and that this erosion increases markedly during solar storms. The mechanism is electrical: the magnetic field carried by the solar wind, as it passes by Mars, generates an electric field that accelerates ions in the upper atmosphere and expels them into space. Repeated over geological ages, that drip is enough to turn a once mild, wet climate into the cold desert with thin air that we see today.